How safe is to keep some BTC in the coldwallet for 20 years paying no attention to crypto world?
Not safe as others have suggested. I agree with Trezor as one of the safest hardware wallet you can get. It maybe expensive, but you can get your money's worth. And as far as not paying attention to crypto world is concern, for me its not also good idea. You have investments, why not pay attention from time to time to see if your investment has profit or not? The only thing can someone will make to do such, is that your are investing what you can afford to lose, so it doesn't matter for you to look at what is current price or what is going on the crypto world.